The multiple forms of cytochrome P-450 regulate the metabolism of a wide variety of xenobiotics and endogenous compounds in vivo. With the aid of monoclonal antibodies, we have demonstrated that human placental microsomes and lymphocytes contain more than two different forms of cytochrome P-450. We have also used monoclonal antibodies to show that murine 7-ethoxycoumarin deethylase behaves the same as that of human placenta. The monoclonal antibodies that inhibit enzyme activity can be used to specifically phenotype cytochromes P-450 containing specific antigenic sites in different tissues and species. These are examples of how monoclonal antibodies to cytochrome P-450 can be used as a tool for classifying P-450s. Making a detailed atlas of the cytochromes P-450 present in different species and tissues will help us in understanding the diversity of cytochrome P-450 and its role in tissue and species susceptibility to carcinogens.